This invention relates to a portable, single-pole (DC) device for detecting an alternating (AC) voltage. More particularly the invention relates to a device with a variable but predetermined threshold for detecting alternating voltage on lines and in installations carrying, or operating on, high voltage or very high voltage, especially above 1,000 volts, or for verifying the absence of voltage.
In order to avoid serious accidents in the course of operation, construction or maintenance work on high-voltage and very-high-voltage installations, strict safety rules are to be observed.
Work on high-voltage and very-high-voltage installations must be done only after these installations are "dead", i.e., cut off from any source of energy.
They can be considered "dead" only if the effective absence of voltage has been verified.
Then they are short-circuited and grounded.
It is known that the voltage induced in the ground under a 420 kv line can base high as 60 kv. It can be at a still higher level in the case of 750 kv and 1-million-volt lines when carrying current.
A conductor ostensibly cut off from any power supply can be brought to a certain potential by electromagnetic induction or electrical fields in the vicinity of high- and very-high-voltage works. There already are portable testers for the presence or absence of voltage, mounted or insulated poles and including a neon lamp.
These detectors in order to work require the creation of a leak to the ground through the operator, who touches the end of a protective resistor for the neon lamp. These devices are appropriate for low voltage. They are, however, very sensitive to induction, and are most unreliable in the vicinity of operating high-voltage works.
Other conventional devices use a neon lamp or a rare-gas lamp in which the firing of the lamp is detected by a photoelectric cell, for example a photoelectric resistor that controls a sound and/or light generator. Sometimes a spark arrestor is interposed between the line and the electrode of the lamp.
Other conventional devices include resistive voltage dividers that lower the voltage passing through the apparatus to a level compatible with a threshold detector.
And finally, there are conventional devices with a simple spark arrestor, of whose electrodes one is connected to a probe and the other to a metal case forming a Faraday cage.
But in the devices known heretofore for detecting alternating voltage, it is an alternating voltage that is applied to the threshold detecting element.
The operating threshold of these devices is not stable, since the voltage exceeds this threshold only intermittently, in view of the alternations of the grid voltage, which, theoretically, are sine-shaped.
Moreover, the lines and installations carrying high and very-high alternating voltage receive surges and sudden waves that upset the proper operation of these devices, which, furthermore, are sensitive to high-frequency discharges and to the ionization of the air.
The lack of stability of the operating threshold is a source of doubt or errors in the recognition of the cut-off or non-cut-off state of works in the vicinity of operating high- and very-high-voltage installations, which can be the cause of serious electrical accidents.
For the safety of the working personnel, it is indispensable to have reliable devices that can distinguish in a sure and precise fashion an induced voltage from a real voltage on the installations.